Summary of work: We have reported that there is an increase with age in the activity of a mitochondrial DNA glycosylase/endonuclease that recognizes the lesion 8-OH-deoxyguanosine (8-OHdG). The activity of this enzyme increased from 6 to 23 months of age in liver mitochondrial extracts from rats and mice. In contrast, two other mitochondrial enzymes of DNA metabolism which are not specifically involved in the repair of oxidative damage, uracil DNA glycosylase (mtUDG) and AP endonuclease, had no change in activity with aging. In mouse liver, we observed an increase in DNA repair with age for the mitochondrial DNA, while the nuclear DNA repair slightly decreased with age. It is likely that the mitochondrial base excision repair glycosylases are induced by DNA damage that accumulates there with age. We have also investigated the role of the oxoguanine DNA glycosylase 1 (OGG1) in mtDNA repair in mice that are defective in this enzyme. We found that liver mitochondria from the OGG1 knockout mice have no detectable 8OHdG incision activity, demonstrating that the mitochondrial activity is encoded by the same gene as the nuclear enzyme. Mitochondrial DNA from the knockout mice accumulate 9 times more 8OHdG than wt animals. In contrast, nuclear DNA from the same animals have only two time more 8OHdG modifications than controls. These results suggest that OGG1 plays a crucial role in the repair of oxidative damage in mitochondria and is probably the only 8OHdG glycosylase in these organelles. Repair of oxidative DNA damage is carried out by the base excision repair (BER) system. BER can occur through two pathways, long and short patch repair. We investigated the repair patch size during repair of uracil in mitochondria from human cells. Our results show that uracil in DNA is repaired solely by the short patch pathway in mitochondria, while the long patch pathway is extensively used in the nuclear repair of the same lesion. The p53 protein has recently been associated with BER in the nucleus. We investigated whether p53 participates in BER in mitochondria and found that mitochondrial extracts from p53 null mouse liver have normal levels of DNA glycosylase activity and of DNA repair synthesis incorporation. However, after gamma irradiation, the intramitochondrial levels of p53 and repair synthesis are slightly elevated, suggesting that p53 translocation to mitochondria may modulate BER up-regulation in response to stress. To investigate the tissue specificity of BER we measured DNA glycosylase actvities in nuclear and mitochondrial extracts from mouse testis, liver, kidney, muscle, brain and heart. Testis had the highest BER levels in both nucleus and mitochondria, suggesting that BER plays a critical role in maintaining genetic integrity. Our results show that BER levels vary greatly among the different organs. Caloric restriction is a major therapeutic intervention against age associated degeneration. We are studying DNA repair of oxidative DNA lesions in calorically restricted mice to determine whether DNA repair is affected by such dietary changes. We are also subjecting knockout mice, defective in specific DNA repair genes, to caloric restriction. This is another approach to determine whether DNA repair plays a role in this process.